424 PROCEEDINGS OP THE AMERICAN ACADEMY, 



diagrams and their accompanying descriptions. In every case, there is no 

 doubt which is the upper end of the cleavage piece, on which the (110) 

 etch-pits can be seen. This study of amphiboles was undertaken pri- 

 marily to determine the value of etch-figures on cleavage pieces, in order 

 to give a means of orienting those belonging to new varieties which are 

 being so often discovered in allotriomorphic development in crystalline 

 rocks. For this reason, I was especially glad to have access to the 

 many species named in the rather voluminous list of page 379. The con- 

 clusion is that, while etch-figures render possible a certain amount of 

 differentiation in the whole family of amphiboles, they yet have so much 

 constancy, so many analogies in outline, as to furnish a reliable means of 

 determining up and down, right and left, in a new variety. 



Etch-Figures of Orthorhombic and Triclinic Amphiboles. 



There remain two problems which I have set before me for solution 

 by means of etching; one, the crystal system of antliophyllite and 

 gedrite, the other, the comparison of aenigmatite and monoclinic amphi- 

 boles in the matter of cohesions on their prismatic cleavage-faces. 



Etch-Figures on Anthophyllite and on Gedrite (110). 



With no other amphibole did I find so much difficulty in producing 

 and discussing figures of corrosion as on anthophyllite and its near 

 relative, gedrite. By dint of some patience, however, pits were ob- 

 tained on (110) that fully served the purpose. The relatively great 

 resistance of these minerals to the solvent power of hydrofluoric acid 

 was illustrated in every specimen. Although recognizable pits could be 

 seei on P. 1, P. 2, and P. 3 at 3 minutes, they were often exceedingly 

 sn.al (longest diameter about equal to 0.002 mm.) and the amount of 

 material removed in solution was insignificant. They could not, however, 

 be much enlarged by longer immersion, as shown by a number of trials 

 at various exposures up to 20 minutes. Instead, they became gradually 

 lost in an indefinite confused surface of irregular solution. The best 

 figures were furnished by P. 2 after 2 minutes' exposure (see Plate I. 

 Fig. 36a and 366), most of them were very shallow, elliptical in shape, 

 with the longer axes of the ellipses uniformly parallel to the vertical 

 axis of the crystal; tliey were commonly aggregated along lines of 

 cleavage. As shown in the diagram, other pits were considerably larger, 

 rectangular, sometimes wholly black under vertical incidence of the 

 light (Plate I. Fig. 36c), at other times characterized by several visible 



